1. Field of the Invention
This invention relates to a displacement measuring apparatus, and in particular to a displacement measuring apparatus for measuring the displacement of an optical type scale on the basis of an interference signal obtained by causing diffracted lights emerging from a diffraction grating formed on the optical type scale to interfere with each other.
2. Related Background Art
As a method of measuring the displacement of an object to be examined with a resolving power and accuracy of the order of microns or submicrons, there is known an encoder in which a monochromatic coherent light beam is caused to enter a diffraction grating moving relative to a reading head. .+-.1st-order diffracted lights of diffracted lights created by the diffraction grating are taken out and these diffracted lights are superposed one upon the other to thereby form an interference light, and this interference light is received by a light receiving sensor. The the displacement of the diffraction grating is measured on the basis of the periodic output signal from the light receiving sensor.
FIG. 1A of the accompanying drawings shows an encoder of this type described in Japanese Laid-Open patent application No. 63-75518. A monochromatic parallel light emitted from a light source 1 is divided into two light beams (a) and (b) by a beam splitter 2, and the light beams (a) and (b) enter target points P and Q on a difraction grating 5 at a particular angle .theta..sub.1 from opposite directions. The +1st-order diffracted light created at the point P and the -1st-order diffracted light created at the point Q are mirror-reflected by points M1 and M2 on a mirror surface 51 formed by evaporating a metal film or the like on the back of a glass scale plate 50 having the diffraction grating 5 formed on the upper surface thereof, and again enter a point R on the diffraction grating 5 at an angle .theta..sub.2. The light beam subjected to +1st-order diffraction at the point P is again subjected to +1st-order diffraction on the diffraction grating 5, and the light beam subjected to - 1st-order diffraction at the point Q is again subjected to -1st-order diffraction on the diffraction grating 5. These .+-.1st-order rediffracted lights have their optical paths superposed one upon the other and emerge perpendicularly from the diffraction grating 5, and enter light receiving sensors 8A and 8B as interference light beams via a quarter wavelength plate 9, a polarizing beam splitter 12 and polarizing plates 11A and 11B.
FIG. 1B of the accompanying drawings shows an encoder described in U.S. Pat. No. 4,606,645. A monochromatic parallel light emitted from a light source 1 is divided into two light beams (a) and (b) by a beam splitter 2, and the light beams (a) and (b) enter target points P and Q on a diffraction grating 5 at a particular angle .theta..sub.1 from opposite directions. The +1st-order diffracted light created at the point P and the -1st-order diffracted light created at the point Q are mirror-reflected by reflecting mirrors M1 and M2, respectively, and again follow the same optical paths to return to the diffraction grating 5 and enter the points P and Q at an angle .theta..sub.2 and are re-diffracted. The .+-.1st-order re-diffracted lights re-diffracted at the target points P and Q on the diffraction grating 5 travel back along the original optical paths and are superposed one upon the other by the beam splitter 2, and enter a light receiving element 8 as an interference light beam.
In such prior-art encoders, the optical path until the +1st-order diffracted light is re-diffracted differs from the otpical path until the -1st-order diffracted light is re-diffracted and therefore, where use is made of a scale having a diffraction grating formed on a glass substrate, if the thickness and refractive index of the glass substrate are irregular, the difference between the lengths of the optical paths of the +1st-order diffracted light and the -1st-order diffracted light varies in conformity with the detected position of the scale. Accordingly, a change in phase occurs between the diffracted lights passing along the optical paths and this provides a measurement error.